How Can I Efficiently Combine Multiple Arrays in C#?

Alternative Solutions for Efficiently Combining Arrays in C

The provided code snippet exhibits slow performance in copying three arrays into a single three-dimensional array. Here are some alternative solutions to achieve a faster execution:

Utilizing Buffer.BlockCopy

Buffer.BlockCopy is specifically designed for high-speed manipulation of primitive types within arrays. It offers significant performance improvements over similar methods in System.Array. To employ Buffer.BlockCopy, follow these steps:

double[] sortedIndex, sortedInstances, sortedLabels;
double[,] leftnode = new double[sortedIndex.Length, 3];

// Copy sortedIndex into the first column of leftnode
Buffer.BlockCopy(sortedIndex, 0, leftnode, 0, sortedIndex.Length * sizeof(double));

// Copy sortedInstances into the second column of leftnode
Buffer.BlockCopy(sortedInstances, 0, leftnode, sortedIndex.Length * sizeof(double), sortedInstances.Length * sizeof(double));

// Copy sortedLabels into the third column of leftnode
Buffer.BlockCopy(sortedLabels, 0, leftnode, sortedIndex.Length * sizeof(double) + sortedInstances.Length * sizeof(double), sortedLabels.Length * sizeof(double));

Exploiting System.Buffer.memcpyimpl

For optimal performance, you can directly invoke System.Buffer.memcpyimpl, which is the underlying function used by Buffer.BlockCopy. This method requires pointers but is optimized for maximum speed. However, note that it only operates on primitive arrays.

//assuming arrays of primitives and pointers are available
double[] sortedIndex, sortedInstances, sortedLabels;
double[,] leftnode = new double[sortedIndex.Length, 3];

unsafe
{
    fixed (double* pSortedIndex = &sortedIndex[0])
    fixed (double* pSortedInstances = &sortedInstances[0])
    fixed (double* pSortedLabels = &sortedLabels[0])
    fixed (double* pLeftnode = &leftnode[0, 0])
    {
        Buffer.memcpyimpl((byte*)pSortedIndex, (byte*)pLeftnode, sortedIndex.Length * sizeof(double));
        Buffer.memcpyimpl((byte*)pSortedInstances, (byte*)pLeftnode + sortedIndex.Length * sizeof(double), sortedInstances.Length * sizeof(double));
        Buffer.memcpyimpl((byte*)pSortedLabels, (byte*)pLeftnode + sortedIndex.Length * sizeof(double) + sortedInstances.Length * sizeof(double), sortedLabels.Length * sizeof(double));
    }
}

Performance Benchmarking

Empirically, Buffer.BlockCopy and System.Buffer.memcpyimpl exhibit competitive performance. The choice between them is often negligible.

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